Create a new track in Pro Tools. Choose "File - New Track" from the main menu in Pro Tools. This will be your submix track.
Set the input of your submix track to one of the buses in Pro Tools. Click the input on the track and select a bus from the list. For this example, try using Bus 1 for a mono submix. You must choose a pair of buses if you want your submix track to be stereo. If your track is stereo, choose Bus 1 to 2.
Set the outputs of the tracks that will be part of your submix. The outputs of the tracks that will be sent to your submix track must match the bus or buses of the submix input that you set in Step 2. Click the output for all of the tracks that will be part of your submix and select Bus 1 for mono or Bus 1 to 2 if your submix track is stereo.
Arm your submix track for recording. Press the record button on your submix track to make it ready for mixdown.
Press "Record." Once your inputs and outputs are set, press the record button on the transport to begin the mixdown process. When you are finished, press "Stop". Your submix is now complete.